You can absolutely train based on heart rate - but in order to do it the correct way you have to know what your max. heart rate is and then work backwards.The only way to do this is to go and get a VO2 max (or submax) test. From here, you can define zones and train in those zones (different zones have different benefits). You might hear about some formulas to figure out your max heart rate - all those have standard error ranges and when it comes to your heart I would not mess around.

All this being said - pace training is typically adequete. Usually you do heart rate training for special purposes.